WASHINGTON — The same bacteria that cause simple strep throat sometimes trigger bloodstream or even flesh-eating infections instead, and over the years, dangerous cases have increased. Now researchers have uncovered how some strains of this bug evolved to become more aggressive.

The bacterial sleuthing may offer clues for developing a vaccine against group A streptococcus and for detecting and fighting outbreaks.

The reason one person recovers easily from strep throat and another gets an invasive infection has lots to do with individual immune systems. But Monday’s research found genetic changes can spur the germ to produce more of two key toxins, becoming more virulent.

“These increased amounts of toxins are like throwing an accelerant onto the fire,” said Dr. James Musser of the Houston Methodist Research Institute, who led the project being reported in the Journal of Clinical Investigation.

Group A strep causes 600 million illnesses worldwide a year, the World Health Organization estimates, mostly strep throat that in developed countries is easily treated by antibiotics.

Invasive strep isn’t nearly as common but can be rapidly life-threatening. The Centers for Disease Control and Prevention estimates 9,000 to 11,500 U.S. cases of invasive strep occur in the U.S. each year, with 1,000 to 1,800 deaths.

Musser’s team, including researchers from Finland, Iceland and the U.S. National Institutes of Health, set out to learn why invasive strep infections increased in both number and severity in numerous countries in the late 1980s, a shift that persists today.

The researchers mapped the genes of nearly 5,000 samples from group A strep infections collected over several decades. They found some small genetic changes that allowed two strains to start producing higher-than-normal levels of two toxins that damage human cells and help the germ better evade the immune system.